Cellulose fiber reinforced biodegradable films based on proteins extracted from castor bean (Ricinus communis L.) cake

Highlights

• Proteins extracted for castor bean cake have good film forming properties but need be reinforced.

• Cellulose fibers have good affinity to proteins matrix.

• Cellulose fibers improve mechanical properties of films without prejudice to others physical properties.

Abstract

The aim of this study was to develop films based on proteins extracted from castor bean (Ricinus communis L.) cake, reinforced with cellulose fibers for use in agriculture, as bags for planting seedlings. The specific aims was to study the effect of fibers concentration on the mechanical properties, color, opacity, gloss, moisture, solubility in water, water vapor permeability (WVP), microstructure, thermal properties, and chemical structure through Fourier transform infrared spectroscopy (FTIR). Proteins were extracted from castor bean cake in a reactor and then freeze-dried. The cellulose fibers were dispersed in water using a high-speed stirrer. The films were produced by dehydration of film-forming solutions (FFS) prepared with the freeze-dried protein (6 g/100 g FFS), cross linker (5 g glyoxal/100 g protein), plasticizer (30 g glycerol/100 g protein), and fibers (0; 2.5; 5; 7.5; 10; and 12.5 g cellulose fibers/100 g protein). The fiber addition had no effect on thickness, humidity, solubility in water and water vapor permeability of the films. In contrast, an increase in puncture force, tensile strength and elastic modulus, and a decrease in the elongation at break were observed as a function of fiber concentration. The fiber addition also affected color, opacity and gloss of the films. Scanning electron microscopy analysis showed that the cellulose fibers were well dispersed in the film matrix, explaining its effect on the mechanical properties of the films. The analysis by Fourier transform infrared spectroscopy (FTIR) corroborated these results. The main conclusion of this study is that the load of cellulose fibers improved the mechanical properties of films produced with the freeze-dried castor bean cake protein.

Introduction

Environmental concerns associated with plastic waste management have emphasized the importance of developing biodegradable materials to reduce the problems of disposal of plastic waste (Fang et al., 2005). A potential material for the production of biodegradable plastics is the films based on biopolymers such as proteins and polysaccharides, which besides their biodegradable nature, are from renewable resources (Sobral, 2000).

Fibers are used as reinforcement materials in biodegradable films to reduce costs and improve it mechanical properties (Kunanopparat et al., 2008, Salgado et al., 2008). Several studies on composites reinforced with vegetable fibers can be found in literature, including wheat gluten based films reinforced with hemp (Kunanopparat et al., 2008), wheat flour based films reinforced with cotton fiber (Dobircau et al., 2009), wheat gluten and fiber wheat straw based films (Montaño-Leyva et al., 2013), corn starch based films reinforced with keratin, lignin and firs cellulose fibers (Bodirlau et al., 2013), composites of soluble potato starch or corn starch reinforced with sugarcane fibers (Gilfillan et al., 2012), cassava starch based films reinforced with wheat straw fibers (Famá et al., 2009), flax-fiber reinforced PLA biocomposites films (Arias et al., 2012), among others.

Among the vegetable fibers, the cellulose fiber is most privileged in studies on biopolymer based films. A highly purified wood pulp composed of 92–98% cellulose, known as dissolved pulp was used in the manufacture of cellulose-derived products (Wertz et al., 2010), and is the most common raw material of microfibrils cellulose (MFC) that can be isolated by employing well-known mechanical treatment methods such as homogenization, microfluidization, microgrinding, and cryocrushing. The stability of its structure confers low solubility of cellulose in almost all reagents and great mechanical strength to the microfibrils (Agoda-Tandjawa et al., 2010, Rezayati Charani et al., 2013, Silva et al., 2008). The cellulose fiber has been applied to produce composites of polyurethane resin derived from castor bean oil (Miléo et al., 2011), starch based films (Müller et al., 2009), composite films from galactoglucomannans (Mikkonen et al., 2011) and rice flour based films (Dias et al., 2011).

The castor bean (Ricinus communis L.) is an oilseed with high economic value because it presents a well-defined market for the oil. Currently, great interest has been given to castor bean cake generated during oil extraction process for its high protein content, which can reach 40% (Lacerda et al., 2014, Silva et al., 2012, Visser et al., 2011). The castor bean cake may be a viable alternative for the development of biodegradable films, contributing to the success of this biodiesel agribusiness (Makishi et al., 2013, Bittante et al., 2014). But considering that this protein was obtained by solubilization, the major peptide fractions was around 40 and 20 kDa (Lacerda et al., 2014). Then, it could be interesting some degree of modification for increase it molecular weight to guarantee a good film-forming property. Chambi et al. (2014) modified that proteins using tannic acid and produced very workable films. And Makishi et al. (2013) produced films of similar proteins cross linked with glutaraldehyde and glyoxal and observed that glyoxal implied in films with improved mechanical properties and lower film solubility than glutaraldehyde. These behaviors can be explained by the high content of arginine in the protein, which react with glyoxal (Makishi et al., 2013).

These materials could have applications in agriculture, particularly as bags for planting seedlings (Bittante et al., 2014). Conventionally, these bags are produced using low-density polyethylene (LDPE) films, which have good physical properties, principally it high mechanical resistance (tensile strength = 34 MPa) (Chambi et al., 2014). However, it is well known that the LDPE is not biodegradable and its bags are difficult to recycle due to the large amount of organic matter adhered to material.

The aim of this paper was to develop films based on proteins extracted from the castor bean (R. communis L.) cake reinforced with cellulose fibers interesting for using as bags for plants seedling. Specifically, it was studied the effect of fiber concentration on the mechanical properties (puncture and tensile tests), color, opacity, gloss, humidity, solubility in water, water vapor permeability, microstructure, thermal properties, and chemical structure through Fourier transform infrared spectroscopy.